1. Field of the Invention
The present invention relates in general to combustible fuel slurries containing liquid hydrocarbon fuel and particulate carbonaceous material, and more particularly, to the prevention or substantial reduction of the settling of the particulate carbonaceous material in the liquid hydrocarbon.
2. Description of the Prior Art
In recent years, the importance of reducing the dependency of the world upon natural gas and liquid hydrocarbon fuels for its energy has been dramatically demonstrated. While not constituting a complete solution to this problem, attempts have been made to add solid particulate carbonaceous material, such as coal, to liquid hydrocarbon fuels because such particulate carbonaceous materials are known to be far more plentiful than liquid fuels.
The idea of using in place of liquid hydrocarbon fuels such as Bunker C fuel oil, a mixture of such oil and finely divided particles of carbonaceous material such as bituminous or anthracite coal or lignite is an old one. In a book published in 1926, Fuels and Their Combustion by Robert T. Haslam et al (McGraw-Hill, New York), there is a discussion on pages 135 and 136 of "colloidal fuel" which is referred to as an emulsion of solid fuel and oil developed by the Submarine Defense Association during World War I. This reference teaches mixing oil with a solid fuel, which may be any of the coals from lignite to anthracite, or peat, coke, or wood, provided that at least 2/3 of the dry solid fuel is combustible and that the fuel is pulverized so that 95 percent of it will pass through a 100 mesh screen and 85 percent of it will pass through a 200 mesh screen. This reference teaches the use of 30 weight percent of coal, 1.5 to 1.2 percent of "fixateur" and the remainder fuel oils, such as pressure-still oil or tar or coal tar. It teaches the use as "fixateur" of lime-rosin-grease (made by heating 83.5 percent oil, 10 percent rosin, 5 percent lime, and 1.5 percent water) or one of the coal tar distillates, such as creosol.
A considerably more extensive discussion of "colloidal fuel" appears at pages 226-234 of Fuels and Combustion Handbook, edited by Alan J. Johnson and George H. Auth, published by McGraw-Hill Book Co., New York, in 1951. This reference points out that the term "colloidal fuels" is a misnomer because in common usage, "colloidal solutions" are ones in which the particles are between 0.1 micron and 0.001 micron in mean diameter, whereas in these fuels, there are particles of coal which have been ground so that 100 percent of them will pass through a 100 mesh screen (150 microns) and 90 percent of them will pass through a 200 mesh screen (74 microns).
The Johnson et al reference shows that those skilled in the art have been aware of the advantages of coal-in-oil fuels: their uses makes it possible to preserve petroleum resources, obtain better use of storage space, permit disposal of fines and low rank coals, etc.
This reference also points out that the behavior of a particular coal-in-oil fuel in respect to settling depends on a number of factors. If the fuel can be prepared constantly at the site of use so that there is a minimum of storage time, stabilization behavior of the coal-in-oil fuel is not important. If stirring or pumping to provide circulation can be used, again there is not much of a problem. Some mixtures remain stable for months without any additional treatment, particularly when the coal particles are fine, the concentration of the coal is relatively high, and the oil is relatively viscous and/or possesses a high specific gravity. Although it is desirable to use a relatively viscous oil, since this promotes stability, the coal-in-oil mixture must not be permitted to become too viscous, because this gives difficulty in connection with pumping the fuel.
The Johnson et al reference also discusses the matter of stabilizers.
" . . . it is a consensus that, with careful attention to a selection of fuels, pulverization, mixing, and storage, stabilizers can and should be avoided in most cases." PA0 1. The coal/oil mixture is poured into a 10 centimeter long by 25 millimeter diameter plastic tube up to about 9 centimeters in height. The top of the tube is then closed with a rubber stopper. PA0 2. The tubes are stored at the temperatures and for the time intervals stated in the examples before analysis. PA0 3. After storage, the sample is cooled and then frozen. PA0 4. The coal/oil mixture is then sliced into five sections of equal length. The coal content is analyzed by dissolving individual sections in warm toluene and filtering it through a piece of No. 1 Whatman paper under water aspirator vacuum. The coal is washed with more toluene repeatedly until the yellow color (oil) disappears from the filter paper. The coal and the filter paper are then dried for two hours in an 80.degree.-100.degree. C. oven and weighed.
The reference cites the work of Aimison Jonnard, "Colloidal Fuel Development for Industrial Use", Bulletin 48, Kansas State College, Manhattan, Kans., January 1946, reporting Jonnard's testing of 148 stabilizing agents. Jonnard "concluded that spent alkylation acid was the only one (of the stabilizers tested) with commercial possibilities."
For reasons set forth above, there is considerable renewed interest in the possibility of extending and/or supplementing liquid fuels with solid fuels. Numerous approaches have been taken to the problem of combining a solid particulate carbonaceous material with a liquid hydrocarbon fuel. It has become apparent to those skilled in the art that, if an effective stabilizing agent is found, the usefulness of the concept of using coal-in-oil fuel is greatly improved.
U.S. Pat. No. 3,907,134, issued Sept. 23, 1975 and 4,082,516, issued Apr. 4, 1978, to Grant W. Metzger, disclose the combination of solid particulate carbonaceous material such as powdered coal, a liquid hydrocarbon fuel such as Bunker C (No. 6) fuel oil, a stabilizing agent, preferably starch, and a viscosity reducing agent, preferably a detergent, more preferably soap, in the '134 patent and anionic surfactants in the '516 patent.
U.S. Pat. No. 4,090,853, issued May 23, 1978, to Clayfield et al, discloses a coal in liquid hydrocarbon fuel product which includes water as a stabilizer and may be further stabilized by the addition of small amounts of surfactants such as anionic surfactants.